1. Field of the Invention
The present invention relates generally to an FM digital data receiver using IS-95 dual mode in which the receiver is selectively operable in either FM or code division multiple access (CDMA) in mobile communication systems. More particularly, the present invention, when continuously receiving a plurality of digital data frames in voice channel mode, relates to an FM digital data receiver for receiving a message without losing the message being repeated in said digital data frame, and a method for receiving the message.
2. Description of the Related Art
Analog mobile communication systems, such as AMPS (Advanced Mobile Phone Service) system, are still being used broadly in the region of North America. Meanwhile, the demand of digital mobile communication systems has also been gradually expanding. In fact, several mobile telephone operators have been gradually changing their analog mobile communication systems to digital mobile communication systems, with the result that some users can not communicate in some areas because different modes of communication are used. Therefore, there have been requests for a dual mode mobile phone being capable of communicating with both analog and digital mobile communication systems. The manufacturers or suppliers of mobile terminals have recently developed a dual-mode mobile telephone to meet the demand of the mobile telephone operators.
In a conventional analog mobile communication system, the demodulation of received FM communication signals is routinely performed using analog processing techniques. However, methods which allow analog communication signals to be processed using digital signal processing techniques have recently been developed. These techniques, such as quadrature detection processes, have enabled analog mobile communication systems to transmit both voice and character message data.
FIG. 1 is a block diagram showing the structure of a digital data receiver for AMPS adapted for dual mode operation.
Referring to FIG. 1, digital data receiver 1 includes an antenna 10 receiving I and Q channel information signals transmitted from a dual mode transmitter (not shown), and analog receiver 11 processing said I and Q channel information signals received through the antenna 10. Baseband I and Q signals being processed in said analog receiver 11 are provided to analog to digital converter (ADC) 12 through each output line 18 and 19 of the receiver 11. Within the ADC 12, baseband I and Q signals are digitized to form 8-bit in-phase (I) and 8-bit quadrature-phase (Q) samples. The 8-bit I and Q samples are input to I/Q FM demodulator 13. Within the I/Q FM demodulator 13, baseband I and Q signals are converted into demodulated frequency signals. The demodulated frequency signals are provided to both an audio decimation filter 15, and to a data recovery unit 14, respectively.
After the demodulated frequency signals are filtered within said audio decimation filter 15, the demodulated frequency signals are provided to a vocoder (not shown) for recovery of the received audio information. Contemporaneously, the data recovery unit 14 extracts symbol synchronization and message word signals from the demodulated frequency signals. These extracted signals are decoded by the data recovery unit 14, and then provided to a microprocessor 17. The microprocessor 17 displays the inputted message word signals on a display device (not shown).
FIG. 2 is a block diagram of the structure of a message synchronization circuit incorporated into the data recovery unit 14 in order to synchronize the symbols. An exemplary example of a message synchronization circuit is set forth in Korean Patent Application Ser. No. 1999-003241, filed Jun. 25, 1997, which is assigned to the assignee of this application and is incorporated by reference.
FIG. 3 illustrates the structure of a digital data frame for transmitting data over a forward voice channel (hereinafter FVC) in an AMPS system.
Referring to FIG. 3, a digital data frame includes a total of eleven sub-frames, and each sub-frame is divided into three signal parts. The three signal parts are known as a given dotting sequence signal, a given word sync signal and a message word signal, respectively, wherein each signal part is represented by a plurality of symbols or bits. Especially, the dotting sequence signal of the first sub-frame in the digital data frame comprises a longer signal than the dotting sequence signal of the rest of the sub-frames. Namely, the dotting sequence signal of the first sub-frame is composed of 101 symbols in which 1 and 0 are repeated, while the dotting sequence signal of the rest of the sub-frames are composed of 37 symbols in which are repeated 1 and 0. The word sync signal included in all the sub-frames in the digital data frame is composed of 11 symbols, having the same pattern, such as 11100010010. The message word signal following the dotting sequence signal and word sync signal in each sub-frame of the digital data frame is composed of 40 symbols. The message word signal is generated by a BCH (Boss-Chaudhuri-Hocquenghem) code method, which is used to code data composed of 28 symbols. The message word signal included in each sub-frame is the same. The dotting sequence signal and word sync signal in each sub-frame allows a mobile telephone to synchronize the message word signal that is received continuously.
The structure of the digital data frame of FIG. 3 was adopted by the TIA (Telecommunication Industry Association) as a standard structure, and the receiving technology for receiving the digital data frame is described in U.S. Pat. No. 5,812,607, which was issued to James A. Hutchison et al. (“HUTCHISON '607”). However, the receiving technology described in the '607 patent has been developed under the circumstance that if the space of the received digital data frame is broad enough, and message word signal must be detected at the start time of the digital data frame. Therefore, if the radio channel was in a bad condition, the provability of detecting of the message word signal would decrease. Furthermore, with the increased demand for transmission of short-message service lately, there have been problems in cases of sending the short-message, which usually consists of 40 symbols, by using only one digital data frame. Therefore, to solve these kinds of problems, several U.S. mobile operators, such as SPRINT, have alternatively used the method of transmitting continuously a plurality of digital data frames. Namely, if a short-message has more than 40 symbols, they are divided and allotted to a plurality of digital data frames, and then transmitted continuously with the plurality of digital data frames. In the case of continuously transmitting a plurality of digital data frames, a mobile phone should successfully receive the plurality of digital data frames transmitted continuously. However, the receiving technology of Hutchison '607 suffers the problems of not receiving accurately the digital data frames because of its use of the method described below with respect to FIG. 4.
Referring to FIG. 4, the receiving method of a digital data frame with respect to Hutchison '607 is set forth. The digital data frame transmitted from the dual mode transmitter (not shown) is synchronized in the symbol sync circuit and the synchronized symbols are recovered within the data recovery unit 14 of FIG. 1. The synchronized and recovered symbols in the data recovery unit 14 are provided to a dotting signal detector (not shown), and the dotting signal detector detects a dotting sequence signal from the received symbols. If a specific pattern of signal, for example a dotting sequence signal composed of 32 symbols in which 1 and 0 repeat, is detected by the dotting signal detector, a control unit (not shown) operates a timer for a predetermined time. Generally, the timer sets a time corresponding to the length of a digital data frame. Therefore, if the dotting sequence signal is detected and the timer is working, message word registers (not shown) in the data recovery unit 14 store each message word signal of each sub-frame in the digital data frame related to the detection of the dotting sequence signal. If the timer expires, the message word signals are demodulated, and the dotting signal detector repeats the detection of the next received digital data frame.
FIG. 5 shows a problem in the case of using the method of the FIG. 4.
Referring to FIG. 5, there is shown the case in which a dotting sequence signal is detected in the second sub-frame because the dotting signal detector (not shown) in the data recovery unit 14 of FIG. 1 could not detect the dotting sequence signal of the first sub-frame in a digital data frame due to sudden weakness of radio channel strength. In this case, the timer operates for the time corresponding to the length of the digital data frame from the time when the dotting signal detector detects the dotting sequence signal. Therefore, if a plurality of digital data frames are received continuously, the timer terminates during receiving of a digital data frame following the first digital data frame. In the above case, the digital receiver can not be notified when the receiving of the first digital data frame ends. The problem lies in that the digital receiver can not separate the first digital data frame from a next digital data frame that includes a different message word signal. Therefore, even though the digital data receiver receives a next digital data frame having a different message word signal, it determines to receive the same digital data frame continuously so that the message word register stores a different message word signal. As a result, it negatively influences the recovering of the message word signal, and may even prevent receiving next digital data frames.